Electrodes

ABSTRACT

Electrodes particularly adapted for use in gas filled spark gap and discharge containers comprising a material resistant to sputtering and vaporization consisting of at least one nitride of a metal of the group consisting of Hf, Zr and Ta and having an O2 content bound as an oxide or oxynitride of less than 5 weight percent and a content of further impurities of less than 1.5 weight percent.

C United States Patent 1 1 [111 3,742,282

Siegle June 26, 1973 [54] ELECTRODES 3,558,966 1 1971 Hill 313/311 x I 3,356,912 12/1967 Rairden et a1. 313/311 UX 175 1 Inventor: Renmngen Germany 3,523,207 8/1970 Johansen et a1. 313/218 x 2,020,055 11/1935 Friederich 313/311 X [73 l Assignee: Robert Bosch GmbH, Stuttgart,

Germany Primary Examiner-Rudolph V. Rolinec Assistant Examiner-Wm. H. Punter [22] Filed 1971 Attorney-Michael S. Striker [21] Appl. No.: 168,748

[30] Foreign Application Priority Data ABSTRACT Aug. 4, 1970 Germany P 20 38 645.2 Electrod p r i larly dapted for use in gas filled spark gap and discharge containers comprising a mate- [52] U.S. Cl. 313/311, 313/218 rial resistant to sputtering and vaporization consisting [51] Int. Cl HOlj 17/04, H01 j l/02 of at least 6 tride Of a metal of the group consisting [58] Field of Search 313/218, 311, 346 of Zr and Ta and having an 2 content bound as n oxide or oxynitride of less than 5 weight percent and a [56] References Cited content of further impurities of less than 1.5 weight UNITED STATES PATENTS P 3,441,777 4/1969 Steinitz 313/311 9 Claims, No Drawings 3,098,723 7/1963 Micks 313/311 UX 1 ELECTRODES This invention relates to. electrodes and more particularly to electrodes adapted for use in gas filled spark gap and discharge containers which electrodes are prepared from a material resistant to sputtering and vaporization.

The life span of gas filled glowand spark-gap type of dischargers which are maintained in small sealed spaces is frequently limited due to atomization of the electrode material during the operation thereof. Be

cause of the resulting coating which forms over of the wall of the container, the effective brightness of the apparatus is decreased and/or the resulting build-up of gas in the coating, the pressure and the composition of the filling gas and therewith the electrical values of the gap distance are altered. If the wall coating is electrically conductive and if it has a higher dielectric constant than the actual wall material itself, there results because of the small distances between the electrode and the wall, a disturbance of the electrical field between the electrodes sufficient so that the firing voltage or ignition tension of the spark gap is lowered.

In the main, electrodes of this type have been manufactured of metals, which metal electrodes, however, for all of the usual applications thereof do not have satisfactory properties. Thus this type of electrode undergoes atomization in operation and/or react with the filling gases, withdrawing noble gases and hydrogen from the container so that they cannot always be satisfactorily used. These electrodes also have the disadvantage that on storage in air, as a rule, they immediately start to cover over with an oxide skin, membrane or film which on installation of the electrode, must be removed prior to use.

Materials for use in the preparation of'such electrodes having oxidation stability and erosion stability are described in German patent No. 1,295,855. These materials are manufactured by intimate admixture of one or more of the nitrides of the metals Nb, V, Ta, Ti, Zr, Hf and A1, a heat stable aluminum compound and a metal in finely divided form. The mixture is then subjected to pressing in a mold and sintered in the conventional manner. In the resulting sintered product the metal component forms a coherent grid affording the material mechanical support. The preparation of such materials is accomplished thorough admixing and the further requirement for reduction of particle size, the latter must be less than 1 pm which have proven to be relatively unacceptable for technical applications. Instead these materials rather than being used to form electrodes of the type herein involved are preferably employed in the formation of abrasion resistant tools and bushings.

. A possibility for reducing the evaporation of luminous bodies, for instance, in electrical incandescent lamps is disclosed in German patent No. 437,165. As disclosed, electrodes are prepared of a core material which is composed of a difficultly meltable material and which is enclosed in a jacketing or covering formed of a compound such as tantalum carbide or nitride which undergoes melting only at very high temperatures and which above all, at the thermal light emitting temperatures which amount to more than 2000C only undergoes a slight degree of evaporation which property would not be the case if the core material were not protected.

It is an object of the instant invention to provide a simple and economically feasible method for manufacgree of atomization. It is another objectof the invention to provide the novel electrode produced by the foregoing method.

Still another object of the invention is to provide such electrodes which on the eventual taking up or emission of gases from the discharge space undergo no alteration in their electron discharge.

Theseand other objects and advantages of the invention will be apparent from a consideration of the following disclosure.

In accordance with the invention it has now been found that electrodes prepared on the basis of a material consisting of at least one nitride of a metal of the group of Hf, Zr and Ta and having an 0 content bound as an oxide or oxynitride in an amount of less than 5 percent by weight and having at most a content of other impurities under 1.5 weight percent will undergo only negligible sputtering or vaporization in use.

In contrast to the electrodes prepared from the heretofore proposed materials, in the electrodes prepared on the basis of nitride materials in accordance with the invention, the mechanical strength is relatively unimportant and the operating temperature of the completed electrode lies under 500C.

The most suitable electrode material is one which contains the least possible amount of impurities. These impurities are particularly troublesome in that they sputter more readily or are vaporized more readily at the temperature of the spark starting points than the nitrides present in the electrode or they react chemically with the gases used in the discharger.

As a rule the, not entirely unavoidable formation of oxides and oxynitrides of Zr, Hf and Ta in the working up of the nitrides to form the electrodes should be reduced as much as possible and the oxygen content maintained below 5 weight percent. in Zr nitride electrodes, this is assured as long as the material maintains a yellow appearance.

Tests carried out with spark gaps in sealed gas chambers which are also suitable for use as spark gaps in ignitition or lighting installations, for example in combustion motors have shown that the electrodes in accordance with the invention when operated in an oxygen and water poor atmosphere (concentration under 5 ppm) undergo an outstandingly small degree of sputtering. Most advantageously, the electrodes are used in atmospheres of nitrogen, argon or their mixtures which can additionally contain small amounts of hydrogen (under 20 percent).

Pencil-shaped electrodes having a diameter of 1,5 mm which in accordance with the invention are prepared from ZrN of a 99.5 percent purity were manufactured so that their oxygen content after their completion remained under 3 weight percent. With a spacing of the electrode points of about 5 mm and operating in a nitrogen atmosphere, only in some cases there was deposited on the glass wall spaced 1.5 mm away following in excess of 10" sparks (individual energy mJ, spark ignition voltage 15 kV), a thin whitish to blueish transparent coating, the ignition potential, however, remaining constant.

' The constancy in the electrodes manufactured in accordance with the invention establishes the absence of, any chemical reactions thereof with the named gases.

If the electrodes are not of the desired degree of purity, for instance, if the oxide or oxynitride content is too high, the deposit formed under the same experimental conditions will appear opaque, white and thick. The ignition voltage then as a result thereof falls off to 40 percent from the starting value.

In electrodes prepared for instance from W, refined steel, or Ni and used under the same conditions there is observed after the same number of sparks, a thicker black to brownish black deposit on the wall and a falling off of the ignition voltage, most generally amounting to percent.

An essential advantage of the use of pure nitride as electrode materials is that on storage in dry air without any protective covering, only a just perceptible difference in the working function of the electrons is observed than in the underlying nitrides. Thereby there is avoided the necessity for special handling of the electrodes, as the required electrical stability properties are not interfered with even after installation of the electrodes.

The electrode material in accordance with the invention can advantageously be used as electrodes in spark gap and other discharge installations such as incadescent, glow, and flash lamps as well as in pre-spark gap and ignition installations for combustion motors. For the base materials, no strong restrictions are made. These materials should withstand the mechanical and thermal treatment during fabrication of the electrodes, they should be able to be operative to endure the mechanical and thermal stresses during operation, and they should be sufficiently dense to avoid degassing when inserted into the gas filled spark gas container. in cases where the electrode material is plasma sprayed or soldered or welded onto the base material, it should have roughly the same thermal expansion coefficient.

Base materials and alloys like Fe, Mo, Ta, W, Ni (Fe- Ni-Cr-and Fe-Ni-alloys like Kovar, Vacon, Vacovit), and steel are thus most favourable. For plasma-sprayed ZrN-coatings, Kovar or Vacon are well suited.

As an example, one typical way of production of pencil-shaped electrodes will be described.

The base-material of the electrode is made from a wiretack out of a Fe-Ni-Cr-alloy (Kovar) with a diameter of 1 mm and a length of 30 mm. Only one end of the wire-tack has to be covered with the actual electrode materials. To enlarge the adhesion of that material on the base, the said end is sandblasted up to 3 mm from the end of the wire-tack, using grains of sand of about 0.2-0.5 mm diameter. After which the wire-tack is cleaned in running water and degassed in a wet H,-or H,-N,-stream at 850C for 15 min. The basematerial is now prepared to be covered with the sputtering and vaporisation resistant material. it is placed into a silicon rubber or a metal form with holes so that the tip sticks out 3 mm above the surface of these holders. A powder of e.g. ZrN having a grain diameter of preferably 10-30 pm) is sprayed by a plasma-burner of 25 kW energy consumption on the base-material which rotates around its axes at about 8 cm diameter from the burner. The axes of the burner and the electrode have an angle of about 20 to ensure that the coating is produced mainly on the tip of the wire-tack. The thickness of the resulting coating is no more than 1 mm on the tip, on the surrounding of the tip the coating does not need to be more than 0.1 mm thick. This coating below the tip is necessary, otherwise the spark may start not on the coating, but on the base material which has a much higher sputtering yield than the coating. The spraying, time necessary is about s. After spraying both the electrode and the holder are covered with a layer of the sprayed material and the electrode has to be broken out of this coherent layer, which can easily be done. To avoid melting or bending of the base mate rial, the metal or the rubber holding, intense cooling is necessary by a gas stream from a nozzle using N or inert Ar. This does not reduce adhesion of the coating if the droplets of the powder reach the electrode in a molten state. To avoid oxidation of the sprayed materials, the gases which flow through the burner should likewise be Ar and or N The spraying is done most favorably in a chamber with a N,-or Ar-atmosphere so as to avoid oxidation of the sprayed materials. But spraying in air is possible too. In the latter case, the unavoidable oxide content is reduced by heating the electrode using a radiant heater after spraying in an appropriate atmosphere N, of high pressure for nitrides like ZrN. E.g. ZrN-coatings are heated to about 750C and more in N, of a pressure of at least 1 bar for about 1 min to reduce the O-content by a factor of 2 to less than 3 weight percent. This heating in a N, -atmosphere is necessary in any case for ZrN (and other nitrides of the transition metals) because the nitrides decompose during the spraying process, which results in a deficiency of N and a greyish color of the coating. After the heat treatment described above, the color has changed to a light-yellow and the O-content is below 5 weight percent. By this treatment the nitrides beeome nearly stoichiometric. The adherence of the coating on the base Kovar wire-tack and within the coating is sufficiently good. To detach the coating a tension of more than 2 kp/mm is necessary, no loosening was ever found when the electrode was exposed to accelerations of up to 60g during 2 hours. This example should not be considered as a restriction of the invention in any respect.

I claim:

1. Electrode adapted for use in gas filled spark gap and discharge containers comprising a material resistant to sputtering and vaporization consisting of at least one nitride of a metal selected from the group consisting of Hf, Zr and Ta, said metal nitride having an 0, content bound as an oxide or oxynitride of less than about 5 weight percent and a content of other impurities of less than about 1.5 weight percent.

2. Electrode according to claim 1 wherein said material resistant to sputtering and vaporization is applied as a covering layer onto a base material selected from the group consisting of metals and alloys having substantially the same thermal expansion coefficient.

3. Electrode according to claim 2 having an electrode base material selected from the group consisting of W, Mo, Ta, Ni, Fe-Ni-Cr-alloy, Fe -Ni alloy and steel and contains in part a covering layer of the same materials which form the actual electrode material for the sparks.

4. Electrode according to claim 2, wherein said covering layer has a thickness of about 0.i-l mm.

5. Electrode according to claim 2 wherein said metallic base material is at least in part provided with a covering layer of said materials by plasma-spraying and has an oxygen content of less than 5 weight percent.

in claim 1 comprising reducing the oxygen content and adjusting the nitride content to stoichiometric quantities by heating said electrode at temperatures above 500C in a N containing atmosphere of more than l bar.

9. A process as defined in claim 9 wherein said atmosphere additionally contains H and/or NH 

2. Electrode according to claim 1 wherein said material resistant to sputtering and vaporization is applied as a covering layer onto a base material selected from the group consisting of metals and alloys having substantially the same thermal expansion coefficient.
 3. Electrode according to claim 2 having an electrode base material selected from the group consisting of W, Mo, Ta, Ni, Fe-Ni-Cr-alloy, Fe-Ni alloy and steel and contains in part a covering layer of the same materials which form the actual electrode material for the sparks.
 4. Electrode according to claim 2, wherein said covering layer has a thickness of about 0.1-1 mm.
 5. Electrode according to claim 2 wherein said metallic base material is at least in part provided with a covering layer of said materials by plasma-spraying and has an oxygen content of less than 5 weight percent.
 6. Electrode according to claim 1 wherein said material resistant to sputtering and atomizing is soldered onto said coil layer or welded onto a metallic base material or inserted in it in the form of pins or welded small plates.
 7. Electrode according to claim 1 wherein said base material is selected from the group consisting of Mo, Ta, W, Ni, Fe-Ni-Cr-and Fe-Ni- steel alloys.
 8. A process for producing the electrode as defined in claim 1 comprising reducing the oxygen content and adjusting the nitride content to stoichiometric quantities by heating said electrode at temperatures above 500*C in a N2 containing atmosphere of more than 1 bar.
 9. A proceSs as defined in claim 9 wherein said atmosphere additionally contains H2 and/or NH3. 